This invention relates to a method for electrophotographic copying which comprises forming an electrostatic latent image on a recording medium, developing the latent image by a single-component magnetic toner, and electrostatically transferring the developed toner image onto a transfer sheet and, more particularly, to a method for electrophotographic copying wherein a recording medium, having a low relative dielectric constant and a high insulating property such as an organic photo-conductive medium, etc. is used as the recording medium and an ordinary general purpose sheet of paper is used as the transfer sheet.
As a dry developer for developing an electrostatic latent image formed on a recording medium, a binary developer consisting of carrier particles such as iron particles or glass beads and toner particles such as color-imparting resin particles has been well known. As a method for dry-type development, a cascade method and a magnetic brush method are well known. In most of the presently available dry type copying machines, the aforementioned developing methods and developer are used to obtain copy images, where the toner and carrier particles such as iron particles or glass beads are mixed together, and these two are subjected to tribo-electric charging, and the toner is tribo-electrically charged, and electrostatically attracted to an electrostatic latent image on the recording medium to conduct development. Since the toner has a definite electrostatic charge in that system, the electrostatic latent image on the recording medium can be precisely developed. It is also possible to conduct not only normal development but also inverse development. Furthermore, the electrostaic charge of the toner is retained even after the development, and thus the toner image can be electrostatically transferred to an ordinary general-purpose sheet by corona charging of the opposite polarity. However, in order to satisfactorily conduct tribo-electric charging between the carrier particles and the toner, these two must be mixed in some definite proportions, and thus, a monitoring unit, that is, a device for the so-called toner concentration control, is required, complicating the copying system. Furthermore, as the carrier particles and toner are mixed by agitating for a long period of time, a toner film, that is, a so-called spent, is formed on the surfaces of carrier particles, reducing the tribo-electric characteristic between the toner and carrier. Therefore, the carrier particles whose life has been exhausted by the spent must be disposed as a waste.
To overcome this drawback, a method of development, where no carrier particles are used but only toner particles are brought into the vicinity of or contact with the surface of recording medium, has been proposed. In this method, ferromagnetic fine particles are contained in the toner to impart to the toner a magnetic property of sensing a magnetic field. This method is applied for use with the conventional magnetic brush development. In this case, no carrier particles are needed, and the developing mechanism can be simplified. Thus, the copying machine itself can be reduced in size. This method has been practically applied to a system, in which direct recording is made on specially treated sheets such as zinc oxide sheets or electrostatic recording sheets. The system is proposed, for instance, in U.S. Pat. No. 3,909,458, and is based on the following developing mechanism. That is, a toner containing ferromagnetic fine particles, i.e., magnetic toner, is brought to a vicinity of the surface of a recording medium to induce in the toner an electrostatic charge of the opposite polarity to the electrostatic latent image on the recording medium, whereby the electrostatic latent image can be developed by the toner due to attraction of the induced charge on the toner and the electrostatic charge on the surface of the recording medium by the electrostatic force based upon the Coulomb's force. The toner thus must have a resistivity so reduced as to readily induce the electrostatic charge in it. However, the system so far desired is not of the type of direct recording on a specially treated sheet as mentioned above, but of the type of indirect recording, that is, a system wherein a recording medium serving as a master is repeatedly used, and after the each development of recording medium, the developed toner image is transferred onto an ordinary general-purpose sheet of low electric resistance.
However, when the afore-mentioned magnetic toner for direct recording is employed in said system involving transfer, development can be satisfactorily carried out because of the low resistivity of the toner, but a is encountered in the transfer step, resulting in an unclear transfer image. Therefore, this application is not practical.
To overcome such in the transfer, attempts have been made to suitably control the resistivity of the magnetic toner. Particularly, in order to make electrostatic transfer onto the conventional transfer sheet by corona charging, several attempts have been proposed for increasing the resistivity of the magnetic toner (as disclosed in Japanese Laid-open Patent Application No. 133028/76, Japanese Laid-open Patent Application No. 51947/77, U.S. Pat. No. 4,121,431 to Nelson and U.S. Pat. No. 4,185,916 to Milton et al). The inventors also found magnetic toners, in which both development and transfer could be satisfied at the same time, by restricting relative dielectric constant of toner to an appropriate range in addition to the resistivity of toner (as disclosed in Japanese Laid-open Patent Application No. 129357/80, Japanese Laid-open Patent Application No. 129358/80, and Japanese Laid-open Patent Application No. 129356/80). These magnetic toners have a resistivity within a range between 10.sup.9 and 5.times.10.sup.15 .OMEGA..cm and a relative dielectric constant within a range between 2 and 5. Such a magnetic toner could make satisfactory development and transferred image which the conventional magnetic toner had not produced. However, successive extensive studies revealed that the magnetic toner can make satisfactory development and transferred image when an inorganic light-sensitive material having a high relative dielectric constant such as selenium or zinc oxide is used as the recording medium, but when a recording medium having a low relative dielectric constant and high insulating property such as an organic photo-conductive medium or Mylar as used, the transfer efficiency of toner to the ordinary sheet is reduced, so that a satisfactory transferred image cannot be obtained. Therefore, when a high insulating recording medium as mentioned above is used, it is in current practice to use a specially treated sheet having a high electric resistance as the transfer sheet to increase the transfer efficiency of the tonor. The afore-mentioned organic photo-conductive material has such merits as easy preparation, an ability to form a photo-conductive film and low cost, and has a possibility to be replaced with the conventional selenium or zinc oxide photo-sensitive material. However, a satisfactory magnetic toner for the ordinary sheet transfer, which is applicable to said organic photo-conductive material, has not yet been developed.